A common architecture for a light-emitting device includes an anode, which acts as hole injector; a hole transport layer disposed on the anode; an emissive material layer disposed on the hole transport layer; an electron transport layer disposed on the emissive material layer; and a cathode, which also acts as electron injector, disposed on the electron transport layer. When a forward bias is applied between the anode and cathode, holes and electrons are transported in the device through the hole transport layer and electron transport layer, respectively. The holes and electrons recombine in the emissive material layer, which emits light.
When the emissive material layer includes an organic material, the light-emitting device is referred to as an organic light emitting diode (OLED). When the emissive material layer includes nanoparticles, sometimes known as quantum dots (QDs), the device is commonly called either a quantum dot light emitting diode (QLED, QD-LED) or an electroluminescent quantum dot light emitting diode (ELQLED).
In order to include QLEDs in multicolor high-resolution displays, different manufacturing methods have been designed. These methods are based on disposing three different types of QDs in three different regions of a substrate such that they emit (through electrical injection, i.e., by electroluminescence) at three different colors: red (R), green (G), and blue (B). Sub-pixels that respectively emit red, green, or blue light may collectively form a pixel, which in-turn may be a part of an array of pixels of the display.
U.S. Pat. No. 7,910,400 (Kwon et al., published Mar. 22, 2011) describes that QD films can be made more uniform using wet-type film exchanging ligand processes where QDs can be connected to each other using organic ligands with particular functional groups at both ends (e.g. thiol, amine, carboxyl functional groups).
United States Patent Application Publication No. US 2017/0155051 (Torres Cano et al., published Jun. 1, 2017) describes that QDs can be synthesized with polythiol ligands, and can lead to better packing when deposited and further cured by thermal processes.
International Application Publication No. WO 2017/117994 (Li et al., published Jul. 13, 2017) describes that through an external energy stimuli (e.g. pressure, temperature or UV irradiation) QDs which emit different colors can be selectively attached to bonding surfaces. Surfaces and ligands of QDs must contain particular ending functional groups (e.g. alkenes, alkynes, thiols) in order to be selectively strongly bonded to each other through chemistry reactions.
This concept is further expanded in International Application Publication No. WO 2017/121163 (Li et al., published Jul. 20, 2017), where QDs with R, G and B emission colors can be patterned separately using cross-linkable ligands and organic connectors through chemistry reactions that are activated selectively with UV radiations at different monochromatic wavelengths.
Park et al., Alternative Patterning Process for Realization of Large-Area, Full-Color, Active Quantum Dot Display, Nano Letters, 2016, pages 6946-6953 describes that QDs with R, G and B emission colors are patterned combining conventional photolithography and layer by layer assembly. QD layers are deposited selectively on activated (charged) surfaces.